1. Field of the Invention
The present invention relates to a control device of an electric power steering apparatus for applying steering assist force to a steering mechanism of a vehicle, and particularly to a control device of an electric power steering apparatus with high reliability constructed so that a driver does not feel a sense of discomfort at the time when after a microcomputer (including an MPU (Micro Processor Unit), an MCU (Micro Controller Unit), a CPU, etc.) of a control computation device stops a function and stops a steering assist by a low-voltage reset, a power source voltage is restored to a normal state and the control computation device restarts.
2. Description of Related Art
Referring to a general configuration of an electric power steering apparatus as shown in FIG. 13, a column shaft 2 of a steering wheel 1 is joined to tie rods 6 of steering wheels through a reduction gear 3, universal joints 4A and 4B, and a pinion rack mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting steering torque of the steering wheel 1, and a motor 20 for assisting steering force of the steering wheel 1 is joined to the column shaft 2 through the reduction gear 3. Electric power is supplied from a battery 14 to a control unit 30 for controlling the electric power steering apparatus and also, an ignition key signal IGN is inputted to the control unit 30 through an ignition key 11. The control unit 30 computes a steering assist command value I used as an assist (steering assist) command based on steering torque Ts detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on the computed steering assist command value I.
The control unit 30 is mainly constructed of a microcomputer, and general function and configuration of its control device are shown in FIG. 14. FIG. 14 is an example of a function and a configuration of the case of driving and controlling a three-phase (a, b and c phases) brushless motor 40.
The steering torque Ts detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 are inputted to a control computation device 50 acting as a control computation part, and a current command value computed by the control computation device 50 is inputted to a gate driving circuit 43. A gate driving signal formed based on the current command value etc. by the gate driving circuit 43 is inputted to a motor driving circuit 44 made of a bridge configuration of FET, and the motor driving circuit 44 drives the three-phase brushless motor 40 through a breaking device 42 for emergency stop. Each of the phase currents of the three-phase brushless motor 40 is detected by a current detection circuit 45, and motor currents ia to ic of three phases detected are inputted to the control computation device 50 as feed back currents. Also, a rotational sensor 41 such as a Hall sensor is attached to the three-phase brushless motor 40, and a rotational signal RT from the rotational sensor 41 is inputted to a rotor position detection circuit 46, and a rotational position θ detected is inputted to the control computation device 50.
Also, the ignition key signal IGN from the ignition key 11 is inputted to an ignition voltage monitor part 15 and a power source circuit part 60, and a power source voltage Vdd is inputted from the power source circuit part 60 to the control computation device 50 and also, a reset signal RS used for an apparatus stop is inputted to the control computation device 50. The motor driving circuit 44 is constructed of six FETs Tr1 to Tr6 constructing a three-phase bridge, and the breaking device 42 is constructed of relay contacts 421 and 422 for breaking two phases. There lay contacts 421 and 422 are normally turned on, and the present example is constructed so as to break a and b phases in an emergency, but combinations of other phases may be used.
Details of the power source circuit part 60 and the control computation device 50 are configured as shown in FIG. 15. The power source circuit part 60 is constructed of a stabilization device 61 for outputting a stabilized power source voltage Vdd of, for example, 5 V based on the ignition key signal IGN, a low-voltage detection circuit 621 for detecting a low voltage (threshold Th1) of the power source voltage Vdd and a reset generation circuit 622 for outputting the reset signal RS when a low voltage is detected, and a voltage reduction monitoring part 62 is constructed of the low-voltage detection circuit 621 and the reset generation circuit 622.
The threshold Th1 is set in the voltage reduction monitoring part 62 and when the power source voltage Vdd becomes the threshold Th1 or less, the reset signal RS is outputted and a CPU 51 and a peripheral circuit 53 are deactivated.
Also, the control computation device 50 is a microcomputer and is constructed of the CPU 51 for performing the whole control, a ROM/RAM part 52 for storing data, programs, etc., and the peripheral circuit 53 used as an interface with peripheral equipment, and the power source voltage Vdd from the power source circuit part 60 is supplied to the CPU 51, the ROM/RAM part 52 and the peripheral circuit 53, and the reset signal RS is inputted to the CPU 51 and the peripheral circuit 53, and the steering torque T, the vehicle speed V, the motor currents ia to ic and the rotor position θ are inputted to the peripheral circuit 53.
Such a conventional electric power steering apparatus is constructed so as to start an assist (steering assist) after an operation of the control computation device 50 is checked by initial diagnosis when the ignition key 11 is changed from an off state to an on state. As a result of this, a time delay of about one second is caused before the steering assist is started after the ignition key 11 is turned on. Then, even when this behavior is a temporary reduction in the power source voltage Vdd, a RAM value of the inside of the CPU 51 cannot be held due to the reduction in the voltage, so that when the microcomputer of the control computation device 50 stops by a low-voltage reset, in a manner similar to processing at the time of turning on the ignition key 11, it is necessary to perform starting processing such as initial diagnosis before the steering assist is started after the power source voltage Vdd is restored normally, and a time delay of about one second developed.
For example, when after an engine is once started, the engine stops and the engine is again started (second cranking), a power source voltage reduces temporarily during operation of the control device of the electric power steering apparatus. Particularly when a power source (battery 14) deteriorates, the power source voltage reduces greatly and in that case, there is probably a possibility that a microcomputer of the control device stops by a low-voltage reset, and there was a problem that a burden on a driver becomes big until a steering assist is started.
In the CPU 51, the minimum voltage necessary to hold a RAM value of the inside of the CPU 51 is lower than a voltage (threshold Th1) in which a reset of the CPU 51 is generated and when a voltage reduction value is higher than or equal to the minimum voltage necessary to hold the RAM value, there is no change in the RAM value even when the reset of the CPU 51 is generated.
From the fact described above, it is requested that by monitoring time or the extent of a reduction in a power source voltage, the contents of initial diagnosis of restarting of a control computation device (microcomputer) should be limited according to the time and the extent of the reduction in the power source voltage and a speedy steering assist can be started, and it is necessary to reduce a burden on a driver.